An undoped silicon glass (USG) film deposited by a chemical vapor deposition (CVD) process has been widely used in the field of IC fabrication as a shallow trench isolation (STI) gap fill, sidewall spacer, inter-metal dielectric (IMD) or passivation dielectric. For the CVD process, O.sub.3 /TEOS is preferred over SiH.sub.4 due to better gap fill or step coverage capability as well as safety consideration. On the other hand, a sub atmosphere (SA) CVD process is prior to an atmosphere (AP) CVD process and a plasma enhanced (PE) CVD process to serve as the deposition technique for depositing the USG film because of the better balance in the deposition rate and the gap fill capability. Therefore, so far, an SA O.sub.3 /TEOS process is commonly used for the deposition of a USG film.
The SA O.sub.3 /TEOS process, however, still suffers from a drawback that the as-deposited film is porous and inclined to absorb moisture. Therefore, a post annealing procedure will be necessary to densify the film. In the meantime, the within film tensile stress is subject to elevation during the thermal annealing procedure so as to cause the film crack. Therefore, the situation of the USG film during the IC production is generally checked to detect or even prevent the crack of the film, and it is preferred to adopt a non-destructive method.
Several parameters of the within film such as the deposition rate and the HF etching rate have been monitored to determine the crack limit of the film in order to prevent from cracking, but none of them is sensitive enough to serve as an indicator to achieve this purpose. In addition, a painstaking scanning electron microscope (SEM) process can be used to check the situation of the USG film via an off line operation, and the SEM results generally tell the crack situation of the film rather than prevent it.